Remove id and description from mr widget poll serializer

What does this MR do?

Since instances of MergeRequestPollWidgetEntity is squashed with the content of MergeRequestPollCachedWidgetEntity instances, we can remove inheriting from IssuableEntity. It contains title and description fields which are often sizeable and polled every time widget.json is requested. They are invalidated withing the update of a merge request, so we can just rely on the values from cached_widget.json